One additional line of breast cancer cells has been established in continuous culture, making a total of 4. On serial passage these cells continue to exhibit stable karyotypic abnormalities and responsiveness to estrogen and other steroid hormones, but synthesis of alpha lactalbumin can no longer be demonstrated. Oncornavirus expression has not been detected. In parallel experiments with a murine cell line stimulated by dexamethasone, mouse mammary tumor virus expression was specifically inhibited by interferon. The results suggested a block at the level of viral assembly or release from the cell membrane.